Japanese Patent Application No. 10-253526, filed on Aug. 28, 1998, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention is directed to an Amplitude Shift Keying (ASK) modulating circuit, and more particularly to an ASK modulating circuit used in a transmitter.
2. Related Art
Japanese Patent Laid-Open Publication No. Hei. 10(1998)-13475 discloses a conventional ASK modulating circuit. In this ASK modulating circuit, a switching device or transistor is interposed between a power source terminal of an oscillating circuit and a direct current power source. Repeatedly turning the switching device on and off by applying a two digit signal produces an ASK modulated wave at an output terminal of the oscillating circuit.
However, in the foregoing circuit, a floating capacitance depicted with a phantom line (shown in the 10-13475 Document) is generated, such that the oscillating circuit effectively contains a capacitor. So long as the capacitor is not charged to some extent, the oscillating circuit cannot oscillate or fails to emit a carrier wave. Thus, this circuit is not suitable for high speed transmission.
Japanese Patent Laid-open Publication No. Hei. 08(1996)-191218 discloses another ASK modulating circuit which is free from the foregoing drawback. However, this ASK modulating circuit requires, for ASK modulation, additional components such a FET in addition to an oscillator. Thus, this ASK modulating circuit becomes more complex than the foregoing ASK modulating circuit.
It is, therefore, a principal object of the present invention to provide an ASK modulating circuit without the foregoing drawbacks.
In order to attain the foregoing object, the present invention provides an ASK modulating circuit which comprises an oscillation circuit generating a pulse oscillation having a given frequency and a frequency divider. The frequency divider includes a binary counter unit having at least an input terminal, an output terminal, and a reset terminal. The frequency divider is active and inactive when low and high levels of a transmission signal wave, respectively, are applied to the reset terminal thereof. The frequency divider divides the pulse oscillation while the frequency divider is active. As a result, the transmission signal wave is modulated together with a carrier frequency which is identical to the divided frequency.